The present invention relates to a bidirectional semiconductor light-emitting element capable of emitting lights irrespective of the application of a positive or negative bias voltage.
A semiconductor light-emitting element such as a light-emitting diode or a semiconductor laser has characteristics including a compact form, a high efficiency, a long life and so on, and hence it has been used for various purposes as a light source in place of a conventional electric lamp or various tube lasers. However, since such a semiconductor light-emitting element basically uses a carrier injection type diode based on a p-n junction or a metal-semiconductor junction, a forward or a reverse polarity is present when a bias voltage is applied to the device. In addition, for this reason, the device generally exhibits an operational characteristic such as a half wave current-rectifying when an AC voltage is applied.
Owing to such diode characteristics, it is required to install a plurality of light-emitting elements or an external rectifier circuit if the conventional semiconductor light-emitting element is powered by AC.
However, the installation of the rectifier circuit inevitably generates an electric switching loss. Consequently, the use of the conventional semiconductor light-emitting element for an AC light source based on commercial power (AC) leads to a reduction in overall power conversion efficiency due to the loss of an external circuit even if the light conversion efficiency of the device is high.
Furthermore, as the energized polarity of the device is one-directional, if the device is used for information signal transmission (optical transmission), it is necessary to convert an AC electric signal or the like into an electric signal vibrated up and down at a virtual mid-point of a bias, and then enter the converted signal. Thus, there is a problem of forcible consumption of power corresponding to a bias current even at the non-entry time of a signal.
As one of the measures to solve these problems, there is available a method, which uses a plurality of semiconductor light-emitting elements as described above. In other words, if a plurality of semiconductor light-emitting elements arranged in parallel while having the polarities reversed each other, the problems may be solved. However, this method necessitated more than one semiconductor light-emitting elements to be provided, therefore, the whole structure becomes more complicated and the cost becomes higher. Besides, there is a problem of a reduction in the operating efficiency of the individual devices. Further, in order to apply to optical transmission, it is necessary to introduce the lights from two separate devices into one optical transmission path, or use two separate optical transmission paths or other complicated means.
FIG. 18 shows an example of an optical transmission system using two separate semiconductor light-emitting elements. This example is described in Japanese Patent Application Laid-Open No. H3-58532(1991), and designed to reduce the consumption of power by transmitting only pulse edge information for the optical transmission of a digital signal. In FIG. 18, a reference numeral 23 denotes a pulse signal source; 24 a coupling capacitor; 29 and 30 semiconductor light-emitting elements; 26a and 26b optical transmission paths; 31 and 32 photodiodes (light receiving devices); and 33 a photodiode parasitic capacity (AC equalizing circuit device). The internal resistors of the capacitor 24 and the pulse signal source 23 constitute an electric differentiating circuit.
The optical transmission system described herein by reference can greatly reduce the consumption of power by transmitting the differential wave of a pulse signal compared with a signal transmission over the full width of the pulse signal. However, as the electrical differential waves (current waves flowing across the capacitor 24) of the signal pulse alternately appear as positive and negative polarity peak waves, conversion into optical signals by using the conventional single semiconductor light-emitting element cannot be made. Therefore, in the example shown in FIG. 18, the two separate semiconductor light-emitting elements 29 and 30 are connected in parallel with reversed polarities, and the positive and negative polarity peak waves of the electrical differential wave are converted into optical signals by using the different semiconductor light-emitting elements. Moreover, the optical signals are transmitted separately, and subjected to electric conversion by the two photodiodes 31 and 32. Then, the converted optical signals are integrated by the capacitor 33 to restore the original pulse signal.
As explained above, in the case of using the conventional semiconductor light-emitting element, it is necessary to use a plurality of devices for optical conversion of an AC electric signal or to perform rectification, bias superimposition and so on, by using an external circuit.
It is an object of the present invention to provide a bidirectional semiconductor light-emitting element capable of performing optical conversion for an AC electric signal by one device.
In order to achieve the foregoing object, according to the invention, there is provided a bidirectional semiconductor light emitting element comprising:
a first semiconductor region of a first type of conductivity;
a second semiconductor region of a second type of conductivity provided on said first semiconductor region;
a third semiconductor region of a first conductivity type provided on said second semiconductor region; and
a semiconductor light emitting layer interposed in said second semiconductor region,
said light emitting layer emitting light by an injection of a tunneling current generated at a reverse-biased p-n junction between said first and second regions under an application of a voltage of a first polarity across said first and third semiconductor regions, and
said light emitting layer emitting light by an injection of a tunneling current generated at another reverse-biased p-n junction between said second and third regions under an application of a voltage of a second polarity, which is opposite to said first polarity, across said first and third semiconductor regions.
According to the invention there is also provided a A bidirectional semiconductor light-emitting element comprising:
a first electrode;
a first p-n junction;
a first semiconductor light emitting layer;
a second semiconductor light emitting layer;
a second p-n junction; and
a second electrode,
said light emitting layer emitting light by an injection of a tunneling current generated at said first p-n junction which is reverse-biased under an application of a voltage of a first polarity across said first and second electrodes, and
said light emitting layer emitting light by an injection of a tunneling current generated at said second p-n junction which is reverse-biased under an application of a voltage of a second polarity, which is opposite to said first polarity, across said first and second electrodes.
According to one preferred embodiment of the invention, there may be provided a bidirectional semiconductor light-emitting element as a diode device, comprising: at least one semiconductor light emitting layer included in an n-p-n junction (or a p-n-p junction); and an electrode in each n-type semiconductor of the n-p-n junction (or in each p-type semiconductor of the p-n-p junction). In this case, an impurity concentration is set so that a tunneling current is generated between bands when each p-n junction of the n-p-n junction (or the p-n-p junction) is a reverse bias, and electricity is conducted to emit a light in any of positive and negative polarities of voltages applied to the diode device.
According to another preferred embodiment of the invention, there may be provided a bidirectional semiconductor light-emitting device which is a diode device, comprising: at least one semiconductor light emission layer included in an n-p-n-p-n junction (or a p-n-p-n-p junction); and an electrode provided in each n type semiconductor outside the n-p-n-p-n junction (or in each p type semiconductor outside the p-n-pn-p junction), wherein an impurity concentration is set so that when a bias voltage is applied to the diode device, a tunnel current may be generated between bands in one of two p-n reverse bias junctions of the n-p-n-p-n junction (or the p-n-p-n-p junction) and the other p-n reverse bias junction may be punched through, and electricity is conducted to emit lights in either case of positive and negative polarities of voltages applied to the diode device.
The p-n junction for generating the tunnel current, an energy difference between an n side conductive band and a p-type valance band in a thermal equilibrium state may preferably be 100 meV or higher.
The semiconductor light emission layers may preferably be constituted of two layers, and the wavelengths of lights emitted from the semiconductor light emission layers are different from each other.
The wavelengths of the emitted lights may depend on the polarities of voltages applied to the diode device.
According to another preferred embodiment of the invention, there may be provided an optical transmission apparatus wherein a differential wave of an electric input signal is applied to a bidirectional semiconductor light-emitting device which is capable of conducting electricity to emit light bidirectionally and which has the wavelengths of emitted lights depending on energized polarities, and an output light of the bidirectional semiconductor light-emitting device is transmitted to thereby perform signal transmission.
According to the semiconductor light-emitting element of the invention, lights can be emitted by a single device with respect to applied voltages of both positive and negative polarities, and a direct connection can be made to an AC power source to set it as an AC light emission source without using a plurality of devices or any external rectifier circuits. Accordingly, it is possible to provide an AC light emission source without any increases of the number of devices or any external loses. Moreover, it is possible to provide a novel application system such as an optical transmission system (described later) for performing optical conversion of an AC electric signal.
As described above, according to the invention, the bidirectional semiconductor light-emitting element can be provided, which is capable of conducting electricity to emit lights in any of positive and negative polarities, and constituting the AC light emission source without using a plurality of devices or any external circuits. In addition, the invention is advantageous in that an AC electric signal can be converted into a light basically without rectification, bias application or other operations, and a novel optical applications system impossible by the conventional device can be provided